Rayleigh scattering from small defects and material non-uniformities in an optical waveguide can lead to significant amounts of optical back reflection, when part of the light propagating in the waveguide is reflected and propagates back towards the input. This back scattering may be particularly significant in high index contrast waveguide systems, where the surface roughness of the waveguide core may scatter light with a higher efficiency. Optical systems built in high index contrast material systems, such as for example silicon-on-isolator (SOI), can have relatively long waveguides and thus exhibit high levels back scattering. This back scattering may be a problem for other components in an optical system, such as for example laser diodes and erbium doped fiber amplifiers (EDFA), for which back reflected light may cause linewidth broadening and/or output power oscillations.
It is often difficult to reduce back scattering through changes in the waveguide fabrication process that improve the surface roughness of waveguides. Thus, a technique that can effectively reduce undesired back reflection in optical waveguide systems and devices with conventional waveguides to an acceptable level without appreciably affecting optical signal quality may be preferred.
Back reflections from a waveguide may be suppressed for example using optical isolators. However, a typical optical isolator is a relatively big component and its use may require additional optical splices/connections. Optical isolators may also require exotic materials, and typically require lenses. All these factors may significantly increase the size and cost of the system.
Back reflections may also be reduced by utilizing specific waveguide modes or by changing the waveguide geometry to reduce sensitivity to the surface roughness. However this does not typically provide sufficient improvement in back scattering, and may impose additional design trade-offs.
Accordingly, it may be understood that there may be significant problems and shortcomings associated with current solutions and technologies for controlling back reflections in optical waveguide systems.